A phone can make and receive telephone calls to and from the public telephone network. It does this by connecting to a network provided by a network operator. The public switched telephone network (PSTN) is the network of the world's public circuit-switched telephone networks. The PSTN includes telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by switching centers, thus allowing any telephone in the world to communicate with any other. Originally the PSTN was implemented as a network of fixed-line analog telephone systems. However, the PSTN is now almost entirely digital in its core and includes mobile as well as fixed telephones.
In addition to telephony, modern mobile phones also support a wide variety of other services such as text messaging, MMS, email, Internet access, short-range wireless communications (infrared, Bluetooth), business applications, gaming and photography. Mobile phones that offer these more general computing capabilities are referred to as smartphones.
Some types of calls are not charged, such as local calls (and Internal calls) dialed directly by a telephone subscriber. In other areas, all telephone calls are charged a fee for the connection. Fees depend on the provider of the service, the type of service being used, i.e., the subscriber's service plan, and the distance between the calling and the called parties. In some circumstances, the caller pays a flat rate charge for the telephone connection and does not pay any additional charge for all calls made.
Many popular mobile telephone plans allow free in-network calling, free mobile-to-mobile calling, free mobile-to-mobile texting, or other discounts based on whether or not the other party one is communicating with is using a mobile phone or whether the other party is using a specific carrier. Discounts may apply to inbound communication, outbound communication, or other criteria.
Currently, the only way to take full advantage of carrier-based discounts such as free mobile-to-mobile, etc. is to either specifically ask all the people one converses with for their phone carrier information or to carefully comb through one's monthly bill to see which numbers qualified under the calling program and remember those for the future. Unfortunately, there is no straightforward method a customer can use to determine whether the person they are communicating with is on a mobile phone or what carrier it is on, hence it is difficult to take full advantage of some calling discounts.